Solar Activity Effects of the Ionosphere: a Brief Review

Review

SPECIAL TOPIC: Space Physics & Space Weather April 2011 Vol.56 No.12: 1202–1211 Geophysics doi: 10.1007/s11434-010-4226-9

Solar activity effects of the ionosphere: A brief review

LIU LiBo1,2, WAN WeiXing1, CHEN YiDing1 & LE HuiJun1

1 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 2 State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100190, China

Received July15, 2010; accepted September 7, 2010

Solar radiation, which varies over multiple temporal scales, modulates remarkably the evolution of the ionosphere. The solar activity dependence of the ionosphere is a key and fundamental issue in ionospheric physics, providing information essential to understanding the variations in the ionosphere and its processes. Selected recent studies on solar activity effects of the ionosphere are briefly reviewed in this report. This report focuses on (1) observations of solar irradiance at X-ray and extreme ultraviolet wavelengths and the outstanding problems of solar proxies, in the view of ionospheric studies, (2) new findings and improved representations of the features of the solar activity dependence of ionospheric key parameters and the corresponding physical processes, (3) possible phenomena in the ionosphere under extremely high and low solar activity conditions that are unique, as indicated by historical solar datasets and the deep solar minimum of solar cycle 23/24, and (4) statistical studies and model simulations of the ionosphere response to solar flares. The above-mentioned studies provide new clues for comprehensively explaining basic processes in the ionosphere and improving the prediction capability of ionospheric models and related applications. ionosphere, solar activity, solar extreme ultraviolet, solar proxy, ionospheric index

Citation: Liu L B, Wan W X, Chen Y D, et al. Solar activity effects of the ionosphere: A brief review. Chinese Sci Bull, 2011, 56: 1202−1211, doi: 10. 1007/s11434-010-4226-9

Solar activity as discussed in the scientific community can The Earth’s upper atmosphere absorbs solar radiation, have different meanings. The first refers to the level of solar resulting in heating, dissociation and ionization, and the activity, indicating the intensity of solar electromagnetic ra- ionosphere is mainly produced via the ionization effect of diation, particularly notable at wavelengths of solar X-rays solar XUV. It is well established that the solar XUV fluctu- and the extreme ultraviolet (EUV) (XUV). A second refers ates regularly and irregularly over timescales from minutes to solar activity events; e.g. coronal mass ejection and solar (flares) and roughly 27 days (solar rotation) to decades proton events. The two meanings have a close relationship (11-year solar cycle), with amplitudes varying up to more as well as distinct differences. Solar activity variability and than 1000 times [1–5]. Larger variability tends to occur at its relationship with the ionosphere have received renewed shorter wavelengths. The variability of the solar activity interest. In this report, we focus on recent studies of the initiates huge variations in the neutral density and tempera- ionosphere under different solar activity conditions and solar ture, ion and electron densities and temperatures, neutral activity effects of the ionosphere. The features of the iono- winds, and electric fields in the ionosphere [6,7]. sphere during solar activity events are out of the scope of our Figure 1 presents the power spectra of the solar 10.7 cm interest here, although they are important issues in space radio flux index (F10.7) and the peak electron density in the physics and may have serious consequences on the geospace ionospheric F2 layer (NmF2) recorded at Wuhan (114.4°E, environment (e.g. geomagnetic storms, ionospheric and 30.6°N) during the period of 1957–2005. The figure evi- thermospheric storms). dently depicts the well-known solar cycle and solar rotation variations in F10.7 and NmF2 at Wuhan. In contrast, NmF2 at *Corresponding author (email: [email protected]) Wuhan varies with components much more complicated

nese scientists.

1 Solar radiation observations and solar proxies

The spectrum of solar XUV radiation and its time evolution are essential in investigating the ionosphere and thermosphere. Solar XUV is nearly totally absorbed before it enters the lower atmosphere. Consequently, solar X-rays and EUV cannot be reliably monitored with ground-based instruments. Solar XUV fluxes were measured by rockets and satellites and estimated using indirect methods. Unfortu- nately, space-based measurements of solar XUV have been intermittent for more than two decades. Solar XUV flux data from the observations of rockets and Atmosphere Explorer satellites have been collected to Figure 1 Power spectra of the solar 10.7 cm index F10.7 and the peak provide a fundamental database for the solar XUV spectrum electron density of the ionospheric F2 layer (NmF2) at Wuhan in 1957– 2005. and empirical models [8]. In recent years, solar XUV observations made by NOAA/GOES, Yohkoh/SXT, SNOE/ SXP, SOHO/SEM, and TIMED/SEE as well as those made than those of F10.7. This implies that the ionosphere is also on board several rockets have enriched our understanding of substantially modified by other sources, such as the tides the solar spectral structure and its temporal variability and planetary waves of the lower atmosphere. [9–13]. Atmosphere Explorer and Solar Heliospheric Ob- The ionosphere is an open system that strongly couples servatory (SOHO) satellites continuously measured solar with the magnetosphere and thermosphere. Various photo- fluxes for the longest durations [14]. Since 1996, solar EUV chemical and chemical reactions and dynamical and elec- fluxes in 26–34 nm and 0.1–50 nm wavelength ranges were trodynamical processes in the system exchange and trans- continuously monitored by the Solar EUV Monitor (SEM) port mass, momentum and energy in a complex manner [6]. spectrometer aboard SOHO, which provides an unprece- The regular and irregular variability of solar activity affects the chemical reactions and physical processes in the system, dented opportunity for ionospheric studies. Detailed infor- and it further strongly modulates the structure and evolution mation on solar XUV observations can be found in pub- of the ionosphere and thermosphere. It is an essential issue lished works [5,15]. in ionospheric physics to study ionospheric characteristics Several empirical models [8,15–17] have been developed and processes under different solar activity conditions. The to quantify wavelength-dependent changes in solar EUV study of solar activity modulation of the ionosphere will irradiance. Among these empirical models, HFG [8], improve our understanding of the ionospheric structure and EUVAC [10] and its high-resolution version HEUVAC its evolution, which is critical in determining ionospheric [11], and EUV97 and its updated version Solar2000 [15] are climatology and long-term trends, and will also deepen our widely used for aeronomic calculations. knowledge of chemical and physical processes in the iono- Unfortunately, solar XUV observations are generally sphere and thermosphere. On the other hand, the solar activ- conducted over short periods and intermittently owing to ity dependence of the ionosphere is one of the core issues their expense. Direct measurements of the solar XUV spec- for ionospheric empirical models. Moreover, it is realized trum and its variability are not available for most times. In that the ionospheric climatology, especially the solar activ- the absence of direct EUV observations, we rely on solar ity effects of the ionosphere, provides an insight into iono- proxies to indicate the intensity of solar activity [5,18]. So- spheric weather problems. lar indices, which are well known and commonly used, in- Over recent years, the solar activity effects of the iono- clude the sunspot number R, the solar radio flux at 10.7 cm spheric parameters have received renewed interest, and wavelength F10.7, He 1083, and the MgII core-to-wing index considerable progress has been achieved. In this brief re- [12,18–22]. These indices are distributed by international view, we focus on four aspects: (1) observations of the solar institutions and are easily available from websites. The irradiance and outstanding problems in solar proxies; (2) longest data series among these indices is the sunspot num- novel and detailed features of solar activity dependence of ber, which has been recorded routinely for over 400 years. ionospheric parameters at different altitudes; (3) the possi- The next longest data series is that of F10.7, which has been ble states of the ionosphere under a condition of extreme almost continuously recorded since the first observation in solar activity; and (4) the response of the ionosphere during 1947. There are at least two justifications for using solar solar flares. Priority is given to works carried out by Chi- proxies to indicate solar activity. Firstly, there is high 1204 Liu L B, et al. Chinese Sci Bull April (2011) Vol.56 No.12 correlation among levels of radiation originating from dif- dian foF2. It is well established that the E-layer critical fre- ferent solar atmospheric regions; that is, the sunspot num- quency foE closely follows the variation of solar zenith an- ber, F10.7, and solar EUV are highly correlated in a statisti- gle; thus, useful solar EUV information can also be esti- cal sense [10,11,15]. Secondly, the sunspot number and mated from foE [28]. Furthermore, it is worth mentioning F10.7 are routinely monitored on the ground. A set of solar that the regional and global mean ionospheric total electron radiation lines (e.g. the He I 1083 nm and Mg II core- content (TEC) [29–31] can well capture the variability of to-wing ratio based on the spectral structure around 280 nm) solar irradiance with the outstanding advantage of the global is recommended to measure the level of solar irradiance coverage of the global positioning system (GPS)-derived [12,13], since the lines have strong correlation with the in- TEC. Similar works involve deducing an ionospheric index tensity of solar EUV flux [5,12,13]. Unfortunately, they are to drive ionospheric models for regional predictions with not widely used yet. data from ionosondes [18,32]. The aforementioned works Doubts have arisen with respect to the feasibility of solar may lead to potential applications in monitoring and fore- proxies as a quantitative representation of solar XUV flux. casting space weather and improving ionospheric models. There have been many attempts to test the effectiveness of It is essential to predict the solar activity accurately, es- different solar indices. Particularly, the yearly average val- pecially the trend of the sunspot cycle. Nowadays, popular ues of the relative sunspot number were widely used to in- prediction approaches include the neural network approach, dicate solar activity in early ionospheric research and em- similar-cycle method, empirical orthogonal decomposition, pirical models. Later works revealed that F10.7 has obvious and wavelet analysis [33]. The properties of upcoming solar advantages over the sunspot number in representing solar cycle 24, especial the intensity and peak time, have been EUV. On the other hand, statistical analysis shows that both forecast. It is interesting and surprising that the predicted the sunspot number and F10.7 follow the amplitude of solar results conflict; some articles predict solar cycle 24 to be a EUV in a nonlinear way [8,10]. Moreover, the solar proxies period of low solar activity [33] while others predict it to and EUV are not well correlated on short-term scales [17] have a peak in solar activity that is 30%–50% higher than (e.g. the day-to-day variation) mainly because solar radia- the peak of the current cycle [34]. The huge discrepancies tion at different wavelengths originates from different indicate that the prediction of solar variability is still an un- sources. Thus, remarkable discrepancies are expected in resolved topic posing a great challenge to researchers. their temporal evolution features. It has long been realized that neither the sunspot number nor F10.7 is ideal for representing solar EUV variability accurately. Recently, modi- 2 Characteristics of the solar activity fied solar proxies (e.g. E10.7 and F10.7P) have been proposed. dependence of the ionosphere E10.7 [15,19] is designed to scale the total EUV energy ar- riving at the top of the Earth’s atmosphere in the units of 2.1 Features around the F2 peak F10.7. On longer temporal scales, E10.7 and F10.7 are highly The ionospheric electron density is highest around the F2 correlated [19], and E10.7 is nearly identical to F10.7. E10.7 peak, and thus, the F2 peak has been the subject of many takes account of the heating effect of the upper atmosphere investigations. The solar activity dependence of ionospheric and is designed to improve thermospheric density modeling F2 parameters has been studied for a long period. The criti- [15]. Another new proxy is F = (F + F )/2, which 10.7P 10.7 10.7A cal frequency of the F2 layer (foF2) or peak density (NmF2) was developed based on daily F and its 81-day moving 10.7 [23,35–44], peak height (hmF2) [6,23,40], TEC [29–31, average value, F10.7A. F10.7P has been used as a solar EUV 45–51], plasma temperature and scale height [40,51], and proxy in solar irradiance empirical models (e.g. HFG and thermosphere winds [52–55], temperature and neutral com- EUVAC) [8,10] and ionospheric investigations. Utilizing positions [55–57] have been recently investigated. These the SOHO/SEM EUV records, Liu et al. [23] further con- studies mainly discussed the complicated trends and sensitiv- firmed that F10.7P represents fairly well the intensity of solar ity of the solar activity dependency of the ionosphere, the EUV flux [8,10]. Therefore, F10.7P is recommended as a better detailed features and physical processes, and how to repre- solar proxy for common use in that it retains the advantages sent those characteristics effectively in applications. of F10.7 of a long-term record and good availability. As mentioned above, the sunspot number R and F10.7 Besides various attempts to retrieve solar proxies directly have been traditionally used in ionospheric studies. Most from solar data, several ionospheric indices have been suc- early studies on the solar cycle effects analyzed ionospheric cessively introduced since the 1950s [18,24–27]. These in- data taken from few stations over a short period. Further- dices are deduced from ionospheric data measured at single more, these results are limited to periods with values of F10.7 stations or over regions. The superiority of ionospheric in- rarely exceeding 200 sfu (solar flux units; 1 sfu = 10–22 W –2 –1 dices over solar indices in representing changes in iono- m Hz ). The relationship between the TEC (or NmF2) and spheric parameters has been validated. Representative F10.7 (or R) was reported to be linear by early works. A ionospheric indices are IG12 [24], the Australian T index, nonlinear dependence of the ionospheric parameters on so- and MF2 [26]. These indices are based on the monthly me- lar proxies was discovered in latter studies [35–38,58,59]. Liu L B, et al. Chinese Sci Bull April (2011) Vol.56 No.12 1205

For example, Balan et al. [58] and many others reported that hancement of the equatorial E×B vertical drift and neutral the TEC and NmF2 linearly increase with solar proxies at winds) and changes in neutral compositions contribute to low and moderate solar activity levels, but the linearity the seasonal difference in the solar activity effects of night- breaks down at a higher activity level at all stations. The time NmF2. Liu et al. [48] provided a quantitative descrip- values of TEC and NmF2 no longer increase (and even de- tion of the three kinds of solar activity effects in the global crease) at higher solar activity levels at some stations, indi- TEC. They illustrated the local time variation and longitu- cating saturation. dinal distributions of the TEC solar activity sensitivity in It is still controversial whether the saturation is a true the four seasons. The saturation is found to cluster in equa- manifestation of the solar activity dependence of the iono- torial anomaly regions. The results are supported by Ma et sphere. If it is true, what is the key factor? Balan et al. al. [61], who used ionosonde NmF2 data to show latitudinal [58–60] showed that there is no ionospheric saturation ef- double peaks in the nonlinear coefficient of NmF2 versus fect for the TEC and NmF2 against solar EUV, and further solar proxies. stated that the saturation feature is the result of the nonlinear Liu et al. [30] analyzed the JPL GPS-TEC data over one variation in the solar EUV with F10.7. In contrast, Liu et al. solar cycle to explore the overall climatological features of [35] found that there is an foF2 saturation effect with EUV the ionosphere. The mean TEC data are averaged globally radiation and it is more significant in equatorial anomaly and over low-, middle-, and high-latitude bands in the regions. They thus postulated that the daily ionospheric southern and northern hemisphere, separately, and both equatorial fountain and pre-reversal enhancement contribute hemispheres together. There is stronger solar activity sensi- largely to the saturation. Liu et al. [23] carried out an analy- tivity in lower latitude bands, and the saturation effect in the sis based on a longer series of daily solar EUV data re- mean TEC versus F10.7 is more pronounced at low latitudes, corded by SOHO/SEM and NmF2 data recorded at 20 while the mean TEC increases more rapidly for higher solar ionosonde stations in the East Asia/Australia sector. The EUV fluxes. These mean TEC data, compared with indi- objective of their work was to quantify the solar activity vidual TEC data, capture more obviously the different sensitivity of daytime NmF2 in the East Asia/Australia sec- time-scale variations in the solar activity because local noise tor and to evaluate the dynamic effects and atmospheric is effectively reduced [1,14,22]. consequences on the solar activity effects of NmF2. Re- The “hysteresis” phenomenon is an unresolved problem markable latitudinal and seasonal differences were illus- [41,46,62,63]. Similar to the “hysteresis” effect for mag- trated in the sensitivity of the response of NmF2 and electron netic materials, foF2 may have different values at the same density at given altitudes to solar XUV [23,31,60]. Liu et al. solar level during different phases of a solar cycle. There is [23] reliably confirmed that the nonlinearity of the solar not yet an accepted explanation for the “hysteresis” effect. EUV against F10.7 itself is not enough to explain the iono- Mikhailov et al. [41] postulated that the effect is associated spheric saturation effect, and found that dynamic and with differences in the geomagnetic activity during the as- chemical processes determine the seasonal and latitudinal cending and descending phases. It is known that geomag- features of the saturation effect. Kane [14] reported that netic activity is generally stronger during the descending solar EUV increased 150% from 1996 to 2000, and NmF2 phase than during the ascending phase. Kane [21] attributed changed 210%–290% at seven stations. He found that the the effect to the delayed response of solar EUV to the solar EUV changes themselves were insufficient to account change in F10.7. Attempts have been made to include the for the observed changes in NmF2. In fact, current iono- possible influence of the historical solar activity state on the spheric theoretical models can reasonably reproduce the ionosphere in ionospheric models [36]. The “hysteresis” observed latitudinal and seasonal differences [23,31,60] if effect, however, is difficult to reproduce effectively in mod- the solar activity effects of neutral compositions and els, which is one of the key problems in long-term predic- chemical and dynamical processes are consistently included tions of the ionosphere. in the models; this further indicates that the solar activity The relationship between foF2 and solar indices is criti- not only determines the photoionization rates but also cal in ionospheric empirical models. A linear pattern is used modulates various factors and processes in the geospace in some models. In contrast, the international reference system. ionospheric model IRI [18] adopts a two-segment linear One exciting finding from recent investigations is that model to simulate the solar activity dependence of electron the solar activity effects of global TEC and nighttime NmF2 density. A threshold of the yearly moving-average F10.7 or have linear, saturation and amplification features. The de- sunspot number is empirically set. In the IRI model, NmF2 tailed trends are found to be dependent on latitude, local increases linearly with the solar index before the threshold time and season [30,37,48]. The solar activity variation in is reached, while it is constant afterward. Renewed investi- nighttime NmF2 at some stations retains the daytime satura- gations validated that the solar activity effect of the iono- tion feature in summer, has a linear feature during the equi- sphere can be well presented by a quadratic pattern [48]. A noxes, and has an amplification feature in winter. Chen et higher-order polynomial does not effectively improve the al. [37] stated that dynamical processes (pre-sunset en- fitting [64]. Thus, we strongly recommend a quadratic 1206 Liu L B, et al. Chinese Sci Bull April (2011) Vol.56 No.12 polynomial be adopted in future ionospheric models. night. With increasing solar activity, the O+–H+ transition On the other hand, changes in thermospheric composi- height moves upward, indicating variations of the topside + + + + tions (e.g. the ratio of [O]/[N2]) substantially affect the ion compositions (O , H , He and N ) at given altitudes ionosphere. The neutral density and temperature in the [70–74]. Therefore, it is possible that light ions are domi- thermosphere increase with the level of solar activity nant at solar minima but give way to O+ ions at solar [56,56] owing to the increase in solar UV heating and ion maxima at some altitudes. In other words, the upper transi- drag. Hedin [65] found higher correlation between the neu- tion height (i.e. the altitude with the same concentrations of tral density and solar EUV than between the neutral density O+ and light ions) becomes higher. Zhao et al. [74] exam- and F10.7. Furthermore, a double-peak latitudinal structure, ined the seasonal and solar activity variations of the plasma similar to that of the equatorial ionization anomaly, is ap- compositions at 800 km and developed an empirical model parent in the thermospheric total mass density measured by for the DMSP plasma density using empirical orthogonal the CHAMP satellite at 400 km [66]. The double-peak functions. Liu et al. [75,76] further studied the topside structure is more apparent for higher solar activity. In addi- plasma density of the DMSP observations and found inter- tion, vibrationally excited N2 greatly increases at solar esting climatological features: in the topside ionosphere maximum, which leads to a higher recombination rate of there is strong annual asymmetry in the yearly variations, O+. Calculation results [43,67] also show that vibrationally and an amplification effect in the solar activity dependence excited N2 may strengthen the nonlinear feature of NmF2 of the plasma density was reported for the first time. with F10.7. Investigation of the solar activity effects of the ionosphere The thermospheric temperature varies with the level of at different altitudes will improve our understanding of solar activity, which is reflected in hmF2 and electron den- variations of the ionosphere and its chemical and dynamical sity profiles. Taking Wuhan for example, hmF2, the bottom- processes. Su et al. [77] analyzed the electron density data side profile parameter B0, and the scale height deduced measured by the Japanese incoherent scatter radar to inves- from ionosonde profile observations increase with solar tigate the altitude dependencies of solar activity variations activity [23,68]. Similar characteristics have been reported of the ionospheric electron density. The observations show in the incoherent scatter radar data recorded at Arecibo and strong altitude differences. Electron density at altitudes be- Millstone Hill [40,51]. Naturally, neutral winds play impor- low 300 km increases nonlinearly with F10.7. The nonlinear- tant roles in the ionosphere changes. During the daytime, ity weakens with altitude. The electron density above 450 enhanced poleward winds decrease the F2 peak height, ac- km is found to vary linearly with F10.7. celerate the recombination loss, and further decrease elec- The topside plasma density profiles can be described tron density. Unfortunately, the direct observations of neu- with hmF2, NmF2, and the plasma scale height [66,76] under tral winds are difficult and not yet available globally. On the the diffusion equilibrium assumption. The scale height is a other hand, the hmF2-derived results [52–54] show that the key parameter for the topside ionosphere [73], and it pro- meridional wind at middle and low latitudes, and especially vides clues in studying the ionospheric structure and its dy- its diurnal amplitude, weakens with solar activity. More- namical processes. Peak parameters (hmF2, NmF2) and scale over, the equatorial vertical drift induces the daytime foun- height can be retrieved from electron profile measurements tain effect, which is the most important dynamical process of the incoherent scatter radars. The profiles are fitted with in the low-latitude ionosphere. Measurements made by sat- a Chapman profile function with an altitude-varying scale ellites and incoherent scatter radars reveal that vertical drift height. Lei et al. [40] and Liu et al. [51] analyzed system- is often strongly enhanced around sunset and has a linear atically the historical records of electron and ion tempera- trend with F10.7 [69]. The sunset pre-reversal enhancement ture and electron density profiles observed by the Arecibo of the vertical drift has important consequences, resulting in and Millstone Hill incoherent scatter radars. Distinct diurnal a much longer duration of the nighttime equatorial anomaly and seasonal variations were found in the ionospheric scale at solar maxima [66]. height. The scale height tends to increase linearly with solar activity, which is associated with the thermal structure and 2.2 Features at different altitudes dynamical processes. They also presented the statistical relationship between the scale height and other parameters, An ionosonde may provide information on the bottomside which may guide modeling of the ionospheric profiles. ionosphere, and GPS receivers register the integrated elec- The increase in plasma density with solar EUV at 800 tron density along the path of the radio signal. The topside km differs from the MU radar results. The former shows an ionosphere is detected by incoherent scatter radars and sat- amplification feature [75]. Moreover, the seasonal variation ellites. The topside ionosphere is closely coupled with the of the plasma density at 800 km is dominated by the annual plasmasphere via chemical exchanges (i.e. O+H+↔O++H, component. The annual asymmetry strengthens at higher + + He +N2→N +N+He). The plasma and energy are injected solar activities [74,76]. It is unexpected that the plasma into the plasmasphere from the topside ionosphere during density at 600 km measured by the ROCSAT-1 satellite has the daytime, and they return to the topside ionosphere at linearity, saturation, and amplification trends as the solar Liu L B, et al. Chinese Sci Bull April (2011) Vol.56 No.12 1207

+ activity increases [66]. There is a distinct equatorial anom- inducing a lower NmF2 and the ratio of O to molecular aly in the plasma density at 600 km around the sunset for ions. The feature is somewhat similar to the G conditions periods of high solar flux, which also indicates enhance- during some geomagnetic storms. On the contrary, there is ment of the equatorial vertical drift. It is interesting that the an amplification effect in the case of extremely high solar electron density saturates at lower altitudes; e.g. at 400 km activity [80], which is similar to that of the mean TEC with [66]. solar EUV [30]. Additionally, the ionosphere was investi- Liu et al. [48,75] and Chen et al. [67] proposed an expla- gated for the period 2007–2009 using measurements of the nation for the three kinds of solar activity dependencies of electron density and thermospheric density made onboard the TEC and electron densities at different altitudes. The CHAMP and GRACE satellites. In particular, the thermos- complicated solar activity effects of the ionosphere can be pheric density at 400 km was found to be unusually low by understood if the variations of the key parameters (NmF2, 30%. These results are fascinating, although they await hmF2 and scale height) are known. According to their publication. scheme, at high altitudes (e.g. 800 km), the most important A novel finding is that, in the absence of solar EUV, the factor is the scale height, which determines the amplifica- value of TEC is negative if the observed fitting relationship tion, and at ROCSAT-1 altitudes, the plasma density is con- is extrapolated to the lowest limit [30]. Naturally, it is im- trolled by all three parameters. The solar activity effect in possible for TEC to be negative. However, it suggests that equatorial regions is strongly modulated by the variation of under extremely low solar activity, the ionospheric physical hmF2, showing remarkable latitudinal differences. processes should be unique and differ from those of the Rich et al. [78] studied the influence of solar rotation on normal conditions. Unknown issues include the solar spec- the topside plasma density and temperature. Compared with trum under these extremes, the state of the ionosphere, and the peak region, the topside ionosphere has stronger solar its dominant physical processes. More importantly, the im- rotation modulation. As mentioned above, there is also pacts on our global climate system need to be determined. strong 27-day modulation of the mean TEC averaged glob- Extreme solar events often accompany strong solar activ- ally and over different latitude bands [29–31]. ity. Many serious events (e.g. the 1989 events, the 2000 Bastille Day event, the October–November 2003 storms [81], and the 1859 event [82]) have been reported in the 3 The ionosphere under extreme solar literature. Although there are innumerable case studies and conditions statistical studies on ionospheric storms, it remains difficult to reliably predict the accurate evolution of the ionosphere The variability of solar activity itself is an elusive topic. when a serious solar event erupts. An extreme storm oc- According to historical sunspot records, virtually no sun- curred on September 1, 1859, with ΔH = −1600 nT recorded spots were observed during the Maunder Minimum period at Mumbai, India. Unfortunately, we do not know what will (1645–1715). Reconstructed records of the cosmogenic iso- happen if an event of such strength or an even stronger tope 14C and tree rings suggest the Grand Maximum, a pe- event reaches the Earth. riod of intense solar activity during 1100–1250 [79]. Fur- thermore, the solar activity during 2007–2009 was extremely low and one of the longest recent solar minima. 4 Response of the ionosphere to solar flares This gives rise to speculation of prolonged low solar activity such as the Maunder Minimum. The international cam- A solar flare is a sudden brightening of radiation in solar paign “Deep Solar Minimum” has been proposed to inves- active regions. Solar flares occur frequently in solar maxima tigate the characteristics of the space environment under years. During solar flares, huge energy is abruptly released conditions of extremely low solar activity and its possible from the Sun, inducing enhancements with varying ampli- influences on global warming. The unusual behaviors and tudes at radio, visible light, and XUV wavelengths with processes of the ionosphere and thermosphere certainly de- durations ranging from about 10 min to hours [83]. Explo- serve close attention. sive radiation propagates at light speed and strikes the Earth Smithtro et al. [80] constructed the solar irradiance under after 8 min. The flares abruptly change the structure and extreme solar activity conditions via separating and extrapo- state of the ionosphere and thermosphere in the sunlit lating coronal and chromospheric emissions. The recon- hemisphere, causing sudden ionospheric disturbances [84]. structed solar irradiance was input to a one-dimensional The disturbance phenomena include sudden cosmic noise global average ionosphere and thermosphere model to simu- absorption induced by sudden electron density enhancement late the upper atmosphere behaviors at different solar activ- in the D region, short-wave fadeouts, sudden phase anoma- ity levels. Their results showed that, at extremely low solar lies, sudden frequency disturbances, magnetic solar flare activity levels, the neutral temperature and hmF2 decline as effects, and a sudden increase in TEC (SITEC) [85]. the solar EUV flux decreases. The lower temperatures im- The ionospheric response to solar flares has been widely ply higher N2 and O2 concentrations at F2 peak altitudes, studied with multiple instruments since the 1960s. Davies et 1208 Liu L B, et al. Chinese Sci Bull April (2011) Vol.56 No.12 al. [84] summarized the sudden disturbance phenomena stant of the neutral atmosphere is large, and thus the ther- during solar flares. Many previous works concentrated on mosphere responds slowly to the rapidly varying XUV in very intense flares observed at a limited number of stations. the course of a flare. Sutton et al. [100] reported the first With the advent and popularity of GPS technology, GPS- measurements of the thermosphere neutral density response TEC became a suitable parameter to effectively monitor to the October 28, 2003 X17.2 and November 4, 2003 X28 SITEC and the overall eruptions of flare radiation. The ad- flares. The density measurements were provided by accel- vantage of GPS-TEC is its high accuracy and its out- erometers on the GRACE and CHAMP satellites at altitudes standing spatial coverage and temporal continuity. GPS- of about 490 and 400 km, X-ray fluxes by GOES-12, and TEC is ideal for monitoring ionospheric disturbances during EUV fluxes by the SEE instrument on TIMED. The exo- solar flares in space weather applications. sphere temperature increases were estimated to be 125–175 K In recent years, many scientists studied global variations and 100–125 K, respectively. It is strange that the thermo- of SITEC during solar flares [86–98]. Representative find- sphere density at low to middle latitudes increased about ings include (1) that GPS-TEC is very sensitive to solar 50%–60% in 72±47 minutes in the first case and 35%–45% flares, and can be used to monitor M+-class flares, (2) that in the second case. The observation challenges our current the conclusion of no correlation between the TEC incre- understanding of the mechanisms governing the thermo- ments and solar zenith angles [85] is incorrect, and (3) that a sphere. strong response is also registered in the thermospheric den- Solar energetic particles (SEPs) also anomalously erupt sity [88]. and accompany solar flares, and are termed solar proton Zhang and Xiao [95] analyzed the GPS-TEC recorded at events [81,83]. The SEP and X-ray effects overlap in the 53 stations for the April 15, 2001 flare and found a negative sunlit hemisphere. Particles emitted during these events correlation of SITEC with the solar zenith angle. Wan et al. seriously affect the ionosphere and neutral atmosphere, es- [86] stated that the time rate of the flare-induced TEC in- pecially in polar regions, resulting in large increases in the crement is proportional to the effective flare radiation flux concentrations of HOx (H, OH, HO2) and NOx (N, NO, and and inversely proportional to the Chapman function associ- NO2) in the mesosphere and stratosphere. They further de- ated with the solar zenith angle. Their result was validated crease the ozone concentration [101,83]. SEPs can penetrate with the GPS-TEC data recorded during the July 14, 2000 the polar D region, which increases the ionization rate there, flare. Chen et al. [89] statistically investigated the X-class resulting in enhanced absorption of radio waves in the D intensive flare events during 1996–2003 and found a nega- region (i.e. polar cap absorption) [102,103]. tive relationship between the TEC increment and solar-terrestrial distance and flare duration. The results of Zhang and Xiao [91–94] show asymmetry of the absolute 5 Summary TEC increment about local noon; that is, the flare-induced TEC increment is greater in the afternoon than in the morn- The ionosphere is mainly produced by solar XUV emissions ing in summer, and the reverse in winter and during the [6] originating in the chromosphere and corona. The evolu- equinox. Moreover, the response is dependent on the flare tion of these emissions is not entirely synchronized [5]. Be- location on the solar disc [83,92]. Zhang et al. [92] found a sides the direct and abrupt response to variations in the larger SITEC for a flare at a lower heliographic longitude. XUV ionization, the ionosphere is controlled by chemical, However, it is unclear how the response of the ionosphere is dynamical and electrodynamical processes, which are also determined by the heliographic position of the flare. modulated by solar activity. Therefore, further investiga- Hitherto, there have been few simulations of the flare ef- tions are required to better understand and represent the fects of the ionosphere. Le et al. [97] developed a simple variability of solar activity and its effects on the ionosphere. model for solar radiation in the course of flares and calcu- Variability in solar activity is embodied in the ionosphere lated the response to the X17.2 solar flare on October 28, globally, while chemical and dynamical processes vary on 2003 using a one-dimensional theoretical ionospheric different spatial scales. Therefore, we should pay attention model. Their calculations validated the strong control of the to not only global characteristics but also local particularity solar zenith angle over the ionospheric response to flares. in the ionosphere variations, particularly during storms. The observed seasonal and local time dependences of the Although both the neutral atmosphere and dynamic factors flare effects of the ionosphere are also reproduced in their play critical roles in the ionosphere variations, long-term modeling. From XUV observations, Huba et al. [98] made and global measurements of the thermosphere and neutral the first global simulation of the ionosphere response for the winds remain insufficient and need to be expanded. In addi- July 14, 2000 flare, and Meier et al. [99] simulated the evo- tion, the polar ionosphere is affected by particle precipita- lution of the ionosphere during this event and evaluated the tion and Joule heating. The contributions from different XUV at different wavelengths. factors need to be separated. There has been recent interest in the thermosphere re- In summary, our understanding of the solar activity sponse to flares. 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